urbancanopy-BLM06 - Boundary-Layer Meteorology(2005 115...

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MEAN WINDS THROUGH AN INHOMOGENEOUS URBAN CANOPY O. COCEAL* and S. E. BELCHER Department of Meteorology, University of Reading, P.O. Box 243, Reading RG6 6BB, U.K. (Received in fnal Form 24 June 2004) Abstract. The mean flow within inhomogeneous urban areas is investigated using an urban canopy model. The urban canopy model provides a conceptual and computational tool For representing urban areas in a way suitable For parameterisation within numerical weather prediction and urban air quality models. Average aerodynamic properties oF groups oF buildings on a neighbourhood scale can be obtained in terms oF the geometry and layout oF the buildings. These canopy parameters then determine the spatially averaged mean wind speeds within the canopy as a whole. Using morphological data For real cities, computations are perFormed For representative sections oF cities. Simulations are perFormed to study transitions between di±erent urban neighbourhoods, such as residential areas and city centres. Such transitions are accompanied by changes in mean building density and building height. These are considered frst in isolation, then in combination, and the generic e±ects oF each type oF change are identifed. The simulation oF winds through a selection oF downtown Los Angeles is considered as an example. An increase in canopy density is usually associated with a decrease in the mean wind speed. The largest di±erence between mean winds in canopies oF di±erent densities occurs near ground level. Winds generally decrease upon encountering a taller can- opy oF the same density, but this e±ect may be reversed very near the ground, with possible speed-ups iF the canopy is especially tall. In the vicinity oF a transition there is an overshoot in the mean wind speed in the bottom part oF the canopy. Mechanisms For these e±ects are discussed. Keywords: Atmospheric boundary layer, Urban canopy model, Urban meteorology. 1. Introduction An ongoing challenge in the modelling oF flow and dispersion in urban areas, as well as in the design and interpretation oF feld experiments, is the heterogeneity oF urban areas. Towns and cities consist oF large buildings with random shapes, sizes and distributions. ²urther complica- tions include changes in mean building density and building height be- tween di±erent parts oF a city, such as residential compared to commercial areas. Yet, many applications, such as the representation oF urban areas in numerical weather prediction and urban air quality models, require that complex urban e±ects be parameterised in a simple, operationally viable way. * E-mail: [email protected] Boundary-Layer Meteorology (2005) 115: 47–68 Ó Springer 2005
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A signifcant simplifcation results iF one can assume some measure oF statistical homogeneity in the underlying urban morphology. This is usually possible over a spatial scale oF order 1 km, known as the neighbourhood scale (Britter and Hanna, 2003). One may then consider spatial averages oF flow quantities over a suitable averaging area. The urban area is represented aerodynamically as a porous medium. Each building exerts a resistive Force,
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urbancanopy-BLM06 - Boundary-Layer Meteorology(2005 115...

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